1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus that has a function of forming an image on both sides of a sheet and a method thereof.
2. Description of the Related Art
Image forming apparatuses disclosed in Japanese Patent Application Laid-open Nos. H11-143138 and 2002-116590 and Japanese Patent Publication No. 3544279, for example, are well-known as an image forming apparatus that has a function of forming an image on both sides of a sheet. To improve operation efficiency in forming an image on both sides of a plurality of sheets, Japanese Patent Application Laid-open No. H11-143138 discloses interleaf control in which images are formed on one sides of two or more sheets between image formations on one side and the other side of a first sheet based on image data stored.
Japanese Patent Application Laid-open No. 2002-116590 discloses a technology to appropriately convey sheets to an image forming unit. According to the technology, at a point that a sheet feeding sensor detects a trailing edge of a sheet, the following sheet is fed by a pick-up roller and a feed roller. At a point that the sheet feeding sensor detects a leading edge of the following sheet, the feed roller is stopped. Time T1 taken from the start of driving the feed roller to feed the following sheet to the detection of the leading edge of the following sheet by the sheet feeding sensor is calculated. The feed roller is driven again after a period T−T1 has elapsed since the feed roller is stopped, where T is a minimum time interval required from feeding of the preceding sheet by registration rollers to feeding of the following sheet.
Japanese Patent Publication No. 3544279 discloses another technology. According to the technology, because a possibility of a jam changes depending on a curl amount of an edge of a sheet when the sheet is reversed, a conveying roller is arranged not to come into contact with a sheet that has a small curl amount when the sheet passes along a path.
In some image forming apparatuses such as those in the technologies as described above, especially in image forming apparatuses having a function of duplex printing, a sheet conveying path sharply changes its conveying direction after an image is fixed for saving space. The image forming apparatus having a function of duplex printing needs to convey a sheet to a double-sided sheet conveying unit after an image is fixed on one side of the sheet. For saving a space, the double-sided sheet conveying unit and an image forming unit are often arranged such that they are projected at the same position, for example, the double-sided sheet conveying unit is arranged downward of the image forming unit. Therefore, the sheet is conveyed to the double-sided sheet conveying unit while being turned by 90 degrees or more to about 180 degrees.
However, if a sheet is turned sharply after an image on the sheet is fixed thereto at the fixing unit, the sheet may jam due to the variation of the states of sheets that are conveyed. Especially, when interleaf control is performed to efficiently form images, a plurality of sheets are moving and temporarily stop in the sheet conveying path. Moreover, the order of sheets being conveyed is complicated. Therefore, when a jam occurs in the sheet conveying path, jam recovery processing is more complicated.
Thus it is desired to suppress a sheet jam between a sheet reversing path and a double-sided sheet conveying path.